The present invention relates to vulcanizable molding materials, and more particularly, those that are peroxide vulcanizable by means of the action of heat. In another aspect, the present invention relates to a method of manufacture of such materials.
In particular, polymers which have no or only few double bonds available in the molecule must be cross-linked with peroxide. The most important peroxide cross-linkable polymer types in this connection are: ethylene vinyl acetate EVA), ethylene propylene copolymerizates (EPM), ethylene propylenediene copolymerizate (EPDM), silicon rubber and polyethylene.
It is possible, with peroxidic cross-linking, to produce C-C bridges between two polymer chains which bridges are responsible for good ageing and compression set of the rubber article on account of their high bonding energy and the short bonding length. Peroxidic cross-linking is therefore also used when extreme requirements (e.g. the use of rubber articles in hot, aggressive media) are placed on the article.
The permanent deformation (compression set) of a mixture (measured according to ASTM D 395) is of great importance as a factor for the stability and accuracy of size and, accordingly, for the use especially of industrial articles, e.g. seals, rollers, hoses, and the like, but also for use on shoe soles. The lower the compression set, the higher the utility of the rubber product (e.g. tightness of a seal).
One possibility for improving this property as well as others is the use of bifunctional organosilicon compounds.
DE-A8 23 28 630 teaches a method for the peroxidic cross-linking of polyethylene in which polyethylene is reacted with an organosilicon compound containing an olefinically unsaturated group in the presence of a compound supplying free radicals. This product is then treated with a silanol condensation catalyst and water.
In practice, vinyl silanes are used practically exclusively for this purpose; however, they have a number of disadvantages: they exhibit a low flash point and, in addition, a high volatility on account of their low boiling point, which proves to be a problem at the temperatures used in the rubber industry, especially when being mixed in, when the silane is not yet bound to the filler used as reinforcing filler. Further disadvantages of the use of vinyl silanes are short ultimate elongations, extremely poor tearing resistances, low energies at break and a poor fatigue behavior in the particular vulcanizates.